'assentive' originates from Latin and English word-formation: ultimately from Latin 'assentire', where the prefix 'ad-' (later assimilated to 'as-') meant 'to/toward' and 'sentire' meant 'to feel or perceive', combined in English with the adjectival suffix '-ive'.

'assentive' developed via the verb 'assent' (from Old French 'assentir' < Latin 'assentire') and was formed in English by adding the suffix '-ive' to produce an adjective meaning 'showing assent'.

Initially related to the action or feeling of assent (from Latin 'assentire'), over time it came to be used as an adjective meaning 'showing or disposed to give assent'.